1. Technical Field
This disclosure relates to a photoacid generating polymer used in photolithography, which is one fabrication process for a semiconductor device; a method for preparing a photoacid generating polymer; and an anti-reflective coating composition comprising the photoacid generating polymer. More specifically, the disclosure relates to a photoacid generating polymer suitable for use in immersion lithography for the fabrication of sub-50 nm semiconductor devices, a method for preparing these photoacid generating polymers, and a top anti-reflective coating composition comprising the photoacid generating polymer.
2. Description of the Related Art
Photolithography is a process for the transfer of a semiconductor circuit pattern formed on a photomask to a wafer, and is one of the most important processes in determining the fineness and integration density of circuits in the fabrication of semiconductor devices.
In recent years, as the integration density of semiconductor devices has increased, new techniques have been developed to adapt to the fine processing required in the fabrication of semiconductor devices. There is an increasing need for fine processing techniques in photolithography processes. As the circuit line widths are becoming finer and finer, the use of short-wavelength light sources for illumination and high numerical aperture lenses is required. Nonlimiting examples of such short wavelength light sources are EUV, F2, ArF and KrF excimer lasers, listed in decreasing order of preference.
A number of studies on the development of sub-50 nm devices have been undertaken. Recent attention has been directed toward the development of suitable processing equipment and materials associated with the use of F2 and EUV as exposure light sources. Several issues arise from the use of EUV and F2 lasers as light sources. Technical solutions for the use of F2 are satisfactory to some extent. However, high-quality CaF2 is difficult to produce on an industrial scale within a short time. Also, since soft pellicles are likely to be deformed upon exposure to light at 157 nm, the lifetime of the light source is short. Hard pellicles incur considerable production costs, and are difficult to produce on a commercial scale due to the nature of light refraction. EUV lasers have their own drawbacks. Suitable light sources, exposure equipment and masks are required for EUV laser use, making their application impractical. Accordingly, the formation of finer high-precision photoresist patterns by using a photoresist adapted to the use of an ArF excimer laser is of importance.
Dry lithography is an exposure system wherein air is filled between an exposure lens and a wafer. In contrast to dry lithography, immersion lithography, which corresponds to an NA scaling technique, is an exposure system wherein water is filled between an exposure lens and a wafer. Since water (with a refractive index (n) of=1.4) is used as the medium for a light source in the immersion lithography, the NA is 1.4 times larger than that of dry lithography using air (refractive index (n)=1.0). Accordingly, immersion lithography is advantageous in terms of its high resolution.
A problem encountered with the fabrication of a sub-50 nm semiconductor device is that alteration of the critical dimension (CD) of a photoresist pattern inevitably occurs during the process for the formation of this ultrafine pattern. These alterations arise from standing waves, reflective notching, and diffracted and reflected light from an underlying layer due to the optical properties of the underlying layer on an overlying photoresist and due to the variation in the thickness of the photoresist. To prevent light from reflecting off the underlying layer, an anti-reflective coating is introduced between the photoresist and the underlying layer. The anti-reflective coating is composed of a material that absorbs light in the range of wavelengths used by the exposure light source. Previous treatments have placed this anti-reflective coating on the bottom, interposed between the underlying layer and the photoresist. With the recent increase in the fineness of photoresist patterns, a top anti-reflective coating (TARC) has also been developed in order to prevent the photoresist pattern from being disrupted by the reflected and diffracted light. Specifically, as remarkable miniaturization of semiconductor devices makes photoresist patterns extremely fine, the use of a bottom anti-reflective coating alone cannot completely prevent the patterns from being disrupted by scattered reflection. Accordingly, a top anti-reflective coating is introduced to prevent the disruption of the patterns.
However, since conventional top anti-reflective coatings for use in dry lithography are water-soluble, they cannot be applied to immersion lithography. In other words, since water is used as a medium for a light source in immersion lithography, it easily dissolves the conventional top anti-reflective coatings. Accordingly, there is need for the development of a top anti-reflective coating for use in immersion lithography that is compatible with immersion lithography. This new top anti-reflective coating must satisfy the following requirements. The top anti-reflective coating must be transparent to a light source and have a refractive index between 1.5 and 1.65, depending on the kind of an underlying photosensitive film (i.e., photoresist) to be used. When the top anti-reflective coating composition is coated on an underlying photosensitive film, it must not dissolve the photosensitive film. The top anti-reflective coating must not be soluble in water upon light exposure, but must be soluble in a developing solution. Finally, the top anti-reflective coating must enable the formation of a vertical pattern for creation of the photoresist.
The above-mentioned stringent requirements make the development of a suitable top anti-reflective coating for use in immersion lithography difficult. One of the sources of this difficulty arises from the conventional top anti-reflective coatings inability to allow for the desired formation of a photoresist pattern. Thus, there exists a strong need for the development of a top anti-reflective coating for use in immersion lithography which is water-insoluble and enables the formation of a vertical pattern upon formation of a semiconductor pattern.